Based on my interpretation of the RAW I would say it is the latter. I know for a fact that you can increase an allomantic power beyond the normal limit of 10 with a spike (see Marsh pg 499) so I don't see why you can not do so after the spike has been added.

It's the latter. The point is you can never increase the *spike's* rating after it's created (subject is dead, power is static). If you look in the Heroes of the Mistborn Trilogy section of Book 3, you'll see Zane is rocking a Steel rating over 10, thanks to...stuff.